1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a semiconductor device having a plurality of semiconductor chips used, for example, for the power amplification of microwave signals.
2. Description of the Prior Art
In general, a high-frequency high-power semiconductor device which comprises a plurality of semiconductor chips mounted on a package and connected parallel to each other, and which operates as one device is known. For example, a GaAs FET (GaAs Schottky barrier Field Effect Transistor) device, which is used for the power amplification of microwave signals, comprises a plurality of GaAs FET chips connected parallel to each other in order to substantially increase the total gate width so that the output power is increased.
With reference to FIG. 1, the above-mentioned microwave high-power GaAs FET device, as an example of a prior art semiconductor device having a plurality of GaAs FET chips, will be explained hereinafter. In FIG. 1, two GaAs FET chips 41 and 42 are mounted on a package base 1 which is made, for example, of copper. The package base 1 is used also as an external source electrode, i.e., grounding electrode. On the package base 1, there are provided a gate electrode section 2 which comprises a ceramic plate 20 and a metalized layer (external gate electrode) 21 formed on the ceramic plate 20 and which comprises an input terminal of the FET device, and a drain electrode section 3 which comprises a ceramic plate 30 and a metalized layer (external drain electrode) 31 formed on the ceramic plate 30 and which comprises an output terminal of the FET device. A gate lead 22 and a drain lead 32, which are used for external connection, are connected respectively to the metalized layer 21 of the gate electrode section 2 and the metalized layer 31 of the drain electrode section 3. Each of the GaAs FET chips or elements 41, 42 is a device having an interdigital structure of the electrodes. In FIG. 1, detailed illustration of the structure of the gate electrode, the drain electrode and the source electrode which are disposed in the interdigital arrangement is omitted. Each of the GaAs FET chips 41, 42 has, for example, four bonding pads 5 of the gate electrode and four bonding pads 6 of the drain electrode. Each of the bonding pads 5 and each of the bonding pads 6 are connected respectively to the metalized layer 21 of the gate electrode section 2 and the metalized layer 31 of the drain electrode section 3 which are both fixed on the package substrate 1, as illustrated in FIG. 1.
In the prior art semiconductor device having the above-mentioned structure, there often occurs a problem in that the device oscillates by itself when the DC power supply voltage is applied thereto. This self oscillation occurs in a frequency range from tens of MH.sub.z to several GH.sub.z, and the mechanism of the self oscillation is supposed as follows. Each active element contained in the chips 41, 42 is electrically connected to the external gate electrode 21 and to the external drain electrode 31 disposed on the package base 1 through relatively long bonding wires 7, and the electrical characteristic of each active element is unequal to each other, but a very small unbalance of the electrical characteristic exists because they are manufactured by the same process and in the same manufacturing condition. Moreover, the chips 41 and 42 are mounted close to each other on the package base 1, so that it is difficult to electrically isolate one of the chips from the other. When the DC bias voltage is supplied to the device under the existence of such unbalances, the transfer of electric power occurs between the chip 41 and the chip 42 through the path including the bonding wires 7 and the electrode 21 or the electrode 31 (for example the path a-b-c-d shown in FIG. 1). Such a transfer of electric power makes the operation of the device unstable, and consequently the above-mentioned self oscillation is induced. The dotted lines with arrows in FIG. 1 illustrate such a transfer of electric power. The self oscillation occurs in the above-mentioned frequency range, and the frequency of the self oscillation varies according to the value of the DC power supply voltage.
Examples of a prior art GaAs FET device having the interdigital structure of electrodes are disclosed in "GaAs Microwave Power FET" by M. Fukuta et al, IEEE Transactions on Electron Devices, Vol. ED-23, No. 4, April 1976, pp 388-394, and "Broad-Band Internal Matching of Microwave Power GaAs MESFET's" by K. Honjo et al, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-27, No. 1, January 1979, pp 3-8.